Animals pojffefs of producing Cold. 48 1 
Upon the revival of literature, the importance of this 
.branch of fcience began very foon to engage the attention 
of philofophers. It could not efcape the general obfervadon, 
In a penetrating and inquiiitive age, when the powers 
of the human mind were employed with fo much , ardour 
and fuccefs in exploring the operations of nature, that the 
element of fire acts a principal part in the fyftem of the 
world ; that by the influence of this element thofe motions 
are begun and fupported in the animal and vegetable king- 
doms, which are eflential to the production ana piefervati 
.on 
of life; and that it is the great agent in 
thofe fucceffive 
combinations and decompoiitions, by which all things on the 
furface of the earth, and probably throughout the univerfe, 
are kept in a continual fluctuation. 
But though the utility of this branch of fcience was per- 
ceived, yet the progrefs that was made in the cultivation of it 
did not keep pace with the opinion which men entertained of 
its importance. Our fenfes inform us, that heat has a real 
exiftence, but they give us no direct information with regard to 
its nature and properties : it is endowed with fuch infinite fub- 
tilty, that it has been called, by a very eminent philofopher, an 
occult quality : by fome it has even been con fide red as an im- 
material being. It is, therefore, with great difficulty that it can 
be made the fubjeCt of philofophical inveftigation ; and hence 
the opinions of men concerning it have been fluctuating and 
various, and the words which exprefs it vague and ambiguous. 
The firfl: ftep that was taken with a view to the cultiva- 
tion of this branch of fcience was the conftruftion of a ma- 
chine for meafuring the variations of fenfible heat ; obferving, 
that heat has the power of expanding bodies, and confidering 
•R r r 2 the 
